The subject invention generally concerns the field of general-purpose oscilloscopes, and in particular, concerns an apparatus and method for allowing such a general-purpose oscilloscope to display digital video signals in a meaningful way.
General purpose oscilloscopes are high input impedance, waveform display and measurement instruments that, as the name implies, are not tailored to viewing any one specific kind of electronic signals, but rather, are equally well employed to view and measure signals in the digital or analog domains. Measurements are commonly made on signals in a range extending from D.C. through high RF.
In contrast, there is a special class of oscilloscopes, known as waveform monitors, that are tailored for viewing and measuring video (i.e., television) waveforms. Waveform monitors include dedicated hardware such as video sync detection circuitry and other television signal related functions, and excel at the task of analyzing television signals, but are so specialized that they can no longer be used to view other kinds of signals. That is, they are not intended to be used in conjunction with a common oscilloscope probe. Moreover, waveform monitors, unlike oscilloscopes, are not high input impedance measurement instruments, but rather are constrained to operate on a 1v p-p signal developed across a 75 ohm impedance.
It is well known that television signals may be encoded at a broadcast station into a standard format, such as NTSC, PAL, or SECAM, and then modulated onto an RF carrier for transmission. However, modern television broadcast stations need to convey video signals internally (i.e., routing of signals from location to location within the station) for processing, and the method of choice is to convert the analog video signals to serial digital form for such internal routing. The television industry serial digital standard most often used for this purpose is ITU-R 601 (hereinafter 601 video).
When a 601 video signal is viewed on a general-purpose oscilloscope, the waveform that one sees is that of an xe2x80x9ceye diagramxe2x80x9d. An eye diagram indicates that an active digital signal is being observed (because one can see that the signal is changing between states, but the eye diagram conveys no content information at all. Being able to recognize video content is important when the probing is being done in conjunction with the repair of video equipment. It is also important when it is necessary to identify a particular signal as coming from a particular video source (such as a particular camera). Finally, being able to recognize video content is important when it is necessary to observe the xe2x80x9canalogxe2x80x9d signal levels that the digital video signal will exhibit upon conversion back to an analog signal. To illustrate this point, consider the following. A technician is probing a test panel comprising several different coaxial cables. Each of the cables is coupled to a 601 video signal source. The 601 video data stream is a 270 M-bit encoded bit stream (sometimes referred to as a xe2x80x9cscrambledxe2x80x9d bit stream by those in video technology). As noted above, viewing a 601 video waveform directly on a general purpose oscilloscope will produce only eye diagrams, and will not provide any useful information that would allow the technician to identify which cables are coupled to which sources.
Television engineers have addressed these problems in multiple ways, each way having drawbacks of its own. In a first arrangement, television engineers at an actual broadcast station combined a nine-inch television monitor, a Tektronix 465 analog oscilloscope, an AJA Digital to Analog Converter unit, and a Tektronix 1760 Video Analyzer, by literally strapping them together on a single oscilloscope cart to provide some modicum of mobility.
In a second arrangement, a Tektronix WFM90 waveform monitor was connected to an SDA601 (again, electrically and physically) to look at 601 video signals.
In a third arrangement, an AJA Digital to Analog Converter unit was coupled to a WFM90 waveform monitor.
These arrangements need to draw AC power from an AC receptacle, and by no stretch of the imagination could these combinations be called xe2x80x9cportablexe2x80x9d, when the size and weight of the combinations of elements are taken into account. In addition, none of the arrangements permits making a hardcopy of the video picture, a hardcopy of a vector scope display, or a hardcopy of the video waveform. Moreover, the 1760 Video Analyzer and WFM90 analog waveform monitor have no capability for saving waveform data for later review.
Even if a more specialized waveform monitor, such as a Tektronix WFM 601, or a Tektronix VM 700 were to be substituted into the above-given arrangements, neither could provide mathematics capability for digital signal processing functions, because neither has the capability to receive and store waveforms (i.e. playback recorded waveforms). Moreover, there would still be no hardcopy capability.
It is important to note that none of the above-noted actual arrangements provides the capability to control all of the features from a single menuing system and remote control (gpib, RS232, or ethernet) port.
What is needed is a truly portable, battery-operated oscilloscope, having capability to monitor 601 video (or other forms of digital video such as HDTV and DVB), convert the signal to analog form, display it on the oscilloscope screen, provide hardcopy output, save and recall both data and setup parameters, and perform digital signal processing on the monitored video signals.
A portable general-purpose oscilloscope employs circuitry for receiving and converting serial digital video signals to composite and component analog signals via a digital to analog converter. The converted analog signals are applied to a signal input of the oscilloscope for monitoring and display. Preferably, the oscilloscope is a digital phosphor oscilloscope. The oscilloscope control menus are arranged to control all of the usual functions of the oscilloscope and also to control the serial digital video circuitry. Apparatus is provided for storing data and waveforms.
In a second embodiment of the invention, circuitry is provided for deriving a trigger signal from a specific digital word of the serial digital bit stream. The trigger signal is applied to the trigger input terminal of the oscilloscope to allow the oscilloscope to capture and display television waveform information around a unique occurrence of an event, as specified by the digital word.
In a preferred embodiment of the invention, the serial digital to analog converter is realized in a module that plugs into a receptacle in the oscilloscope.
In yet another embodiment of the invention, separate modules are provided for 601 video, HDTV signals, and DVB signals.
In a further embodiment, separate modules may be provided for NTSC, PAL, and SECAM television standards.